DESCRIPTION (provided by investigator): Influenza A and B viral infections kill hundreds of thousands of people worldwide each year, costing society billions of dollars in morbidity and disruption. Seasonal influenza annually infects 5 to 20 percent of the population, leading to 200,000 hospitalizations and approximately 36,000 deaths. We are in the midst of an influenza pandemic caused by a novel H1N1 influenza strain that includes swine origin viral DNA. The 2009 Pandemic H1N1 influenza strain is causing excess morbidity and mortality in children and young adults. This study of life-threatening influenza infection in children, designed by an established group of pediatric critical care clinical trial investigators and being conducted during an influenza pandemic, evaluates how the host innate immune response is associated with disease susceptibility, severity and outcome. Our preliminary data revealed that both cytokine storm and innate immunosuppression are associated with mortality in children with influenza infection. Based on this, we hypothesize that infection with the influenza virus triggers hypercytokinemia and innate immunosuppression in a genetically susceptible host leading to severe and sometimes fatal infection. We will test this hypothesis in critically ill children with influenza lower respiratory tract infection. By understanding why children die from influenza, we can better identify targets for clinical trials in influenza treatment and prevention. This study has three specific aims. Aim 1 is to identify cytokines and chemokines present on clinical admission that are associated with illness severity and predictive of death or near death in children with influenza lower respiratory tract infection. Aim 2 is to assess the role of innate immunosuppression in influenza lower respiratory tract infection severity and mortality. To do this, we will serially measure the patient's ability to produce proinflammatory cytokines upon ex vivo stimulation of whole blood with lipopolysaccharide and polyinosinic:polycytidylic acid to allow functional determination of innate immune responsiveness. Aim 3 is to identify associations between children with life-threatening influenza lower respiratory tract infection and polymorphisms in influenza-related innate immunity genes. To achieve these aims, we will serially test important inflammatory and immune mediators from peripheral blood and lung, performing sensitive multiplex viral PCR testing to identify coinfection and subtype the influenza strain, and obtain DNA from patients and biological parents. Our approach will entail engagement of 31 sites across the Pediatric Acute Lung Injury and Sepsis Investigator's (PALISI) Network, a consortium of clinical investigators across North American pediatric ICUs with a proven track record of productivity. Including subjects from prior studies, we will have DNA on 400 children admitted to the ICU with life-threatening or fatal influenza and most of their biological parents. Understanding of the role of the innate immune and inflammatory response in children with life-threatening and fatal influenza infection could provide new targets for future preventive (immune adjuvant) and therapeutic approaches.